1. Technical Field
The present invention relates to a condenser microphone that does not undergo the vibration of an electronic circuit under high acoustic pressure to have an excellent frequency response and generate noiseless high-quality audio signals.
2. Background Art
Microphones are placed near musical instruments for collection of the musical sound generated from the instruments under enormously high acoustic pressure. Condenser microphones are often used near musical instruments because they are usually smaller than dynamic microphones and thus do not interfere with the action and view of the players of the musical instruments. A condenser microphone used to collect the musical sound in a theater is often used for amplification (which sometimes is also referred to as “public address (PA)”) and most of them have unidirectionality. A unidirectional condenser microphone has an opening on the side of the microphone case that is in communication with a rear acoustic terminal of the condenser microphone unit. The opening allows acoustic waves to reach the rear acoustic terminal of the condenser microphone unit.
A condenser microphone requires an electronic circuit, such as an impedance converter, to reduce the output impedance of the microphone unit. This structure differs from dynamic microphones that output signals from both ends of a voice coil disposed in a magnetic field. Although a condenser microphone inevitably includes an electronic circuit, it should be compact as much as possible. Thus, the circuit board on which the electronic components are mounted is installed near the rear acoustic terminal of the microphone unit.
The use of such a condenser microphone near a musical instrument causes acoustic waves with high acoustic pressure to enter the microphone case and vibrate the circuit board. The vibration of the circuit board causes the vibration of the electronic components mounted on the circuit board. This generates extra sounds (noise) other than the target sound to be acousto-electrically converted by the microphone unit. If a film capacitor is mounted on the circuit board, the vibration varies the distance between the films of the film capacitor and causes a fluctuation in the capacitance. The fluctuation in the capacitance causes noise. The vibration of the circuit board also causes the fluctuation in the capacitance and thus noise. Such noise impairs sound quality. Thus, it is desirable to prevent the vibration of the electronic components caused by acoustic waves.
A microphone unit case (hereinafter referred to as “unit case”) has a space in communication with an opening behind the rear acoustic terminal of the microphone unit. The circuit board on which the electronic components are mounted is accommodated in this space. The space functions as an acoustic capacitor. Thus, the acoustic mass of the space resonates with the opening of the unit case. Such resonance impairs the frequency response of the microphone. A large space can readily accommodate the circuit board on which the electronic components are mounted but has reduced impedance of the acoustic mass. Thus, the large space readily causes resonance in the main frequency range for sound pickup.
An increase in the impedance of the acoustic mass requires a decrease in the space. A reduced volume of the space increases the impedance of the acoustic mass and the resonant frequency of the space. A resonant frequency of the space higher than the upper limit of the main frequency range of sound pickup does not impair the directional frequency response in frequency ranges below the resonant frequency.
A known condenser microphone has a filler functioning as an acoustic resistor disposed in the space in the microphone case disposed behind the rear acoustic terminal of the condenser microphone unit (refer to Japanese Patent Application Publication No. 2011-9807). An object of the known condenser microphone is to prevent resonance of the space. The filler used in the condenser microphone according to Japanese Patent Application Publication No. 2011-9807 is sintered plastic having continuous pores. The filler is disposed in the space in the microphone case. The filler functions as an acoustic resistor against the sound waves entering the space through the opening on the side of the microphone case to prevent the resonance of the space.
The condenser microphone according to Japanese Patent Application Publication No. 2011-9807 prevents the resonance of the internal space of the microphone case. Unfortunately, the condenser microphone according to Japanese Patent Application Publication No. 2011-9807 does not accommodate an electronic circuit in a small space in a microphone case and does not have a structure to prevent the vibration of the electronic circuit due to sound waves entering the microphone case.
The inventor filed a patent application for a condenser microphone (refer to Japanese Patent Application No. 2012-148150 (U.S. counterpart to the Japanese Application is U.S. Patent Application Publication U.S. 2014/0003628) that includes: a condenser microphone unit; a microphone case that accommodates the condenser microphone unit, a space connecting to a rear acoustic terminal of the condenser microphone unit, and an opening in a peripheral wall connecting to the space; a volume restrictor that is disposed in the internal space of the microphone case and reduces the volume of the internal space; and an electronic circuit disposed in the internal space of the microphone case and surrounded by the volume restrictor, wherein the volume restrictor also serves as a shield that prevents sound waves entering the internal space through the opening in the microphone case from reaching the electronic circuit.
The condenser microphone according to the copending patent application has a small space in the microphone case in communication with the rear acoustic terminal and microphone case of the microphone unit. The small space barely resonates, and thus contributes to an excellent frequency response. The volume restrictor blocks acoustic waves with high acoustic pressure entering the microphone case not to reach the electronic circuit. Thus, noise is not generated by the vibration of the electronic components.